rpi is based only on win loss record (25% w/l record, 50% your opponents w/l record, 25% your opponents/opponents w/l record)...though now there is also an element of home/away included.

pomeroy rating is like the sagarins and is based on all kinds of calculations including won/loss, home/away, SOS, etc....many computer based ratings also factor in last years rating for the first month or so (to have a starting point)

Both ratings flucuation wildly the first 10 games or so. For example, #1 Florida currently has an rpi of almost 200...we all know that isn't right (especially considering the mocs lost to them by about 50, yet are ranking 155 in the rpi).